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Date:   Thu, 20 Jan 2022 16:12:04 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Kees Cook <keescook@...omium.org>, Arnd Bergmann <arnd@...db.de>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Nathan Chancellor <nathan@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Helge Deller <deller@....de>,
        Daniel Latypov <dlatypov@...gle.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: [PATCH v2] Kconfig.debug: Make DEBUG_INFO selectable from a choice

Currently it's not possible to enable DEBUG_INFO for an all*config build,
since it is marked as "depends on !COMPILE_TEST". This generally makes
sense because a debug build of an all*config target ends up taking much
longer and the output is much larger. Having this be "default off" makes
sense. However, there are cases where enabling DEBUG_INFO for such builds
is useful for doing treewide A/B comparisons of build options, etc.

Make DEBUG_INFO selectable from any of the DWARF version choice options,
with DEBUG_INFO_NONE being the default for COMPILE_TEST. The mutually
exclusive relationship between DWARF5 and BTF must be inverted, but the
result remains the same. Additionally moves DEBUG_KERNEL and DEBUG_MISC
up to the top of the menu because they were enabling features _above_
it, making it weird to navigate menuconfig.

Suggested-by: Arnd Bergmann <arnd@...db.de>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Masahiro Yamada <masahiroy@...nel.org>
Cc: Nick Desaulniers <ndesaulniers@...gle.com>
Cc: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Reviewed-by: Arnd Bergmann <arnd@...db.de>
Reviewed-by: Nathan Chancellor <nathan@...nel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
Signed-off-by: Kees Cook <keescook@...omium.org>
---
v1: https://lore.kernel.org/lkml/20211210000910.3597609-1-keescook@chromium.org/
v2:
 - simplify default vs COMPILE_TEST (Arnd, Masahiro)
 - add Reviewed/Tested-by tags
---
 lib/Kconfig.debug | 141 ++++++++++++++++++++++++----------------------
 1 file changed, 75 insertions(+), 66 deletions(-)

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 5c12bde10996..3be525ad0013 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -208,20 +208,88 @@ config DEBUG_BUGVERBOSE
 
 endmenu # "printk and dmesg options"
 
+config DEBUG_KERNEL
+	bool "Kernel debugging"
+	help
+	  Say Y here if you are developing drivers or trying to debug and
+	  identify kernel problems.
+
+config DEBUG_MISC
+	bool "Miscellaneous debug code"
+	default DEBUG_KERNEL
+	depends on DEBUG_KERNEL
+	help
+	  Say Y here if you need to enable miscellaneous debug code that should
+	  be under a more specific debug option but isn't.
+
 menu "Compile-time checks and compiler options"
 
 config DEBUG_INFO
-	bool "Compile the kernel with debug info"
-	depends on DEBUG_KERNEL && !COMPILE_TEST
+	bool
 	help
-	  If you say Y here the resulting kernel image will include
-	  debugging info resulting in a larger kernel image.
+	  A kernel debug info option other than "None" has been selected
+	  in the "Debug information" choice below, indicating that debug
+	  information will be generated for build targets.
+
+choice
+	prompt "Debug information"
+	depends on DEBUG_KERNEL
+	default DEBUG_INFO_NONE if COMPILE_TEST
+	help
+	  Selecting something other than "None" results in a kernel image
+	  that will include debugging info resulting in a larger kernel image.
 	  This adds debug symbols to the kernel and modules (gcc -g), and
 	  is needed if you intend to use kernel crashdump or binary object
 	  tools like crash, kgdb, LKCD, gdb, etc on the kernel.
-	  Say Y here only if you plan to debug the kernel.
 
-	  If unsure, say N.
+	  Choose which version of DWARF debug info to emit. If unsure,
+	  select "Toolchain default".
+
+config DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
+	bool "Rely on the toolchain's implicit default DWARF version"
+	select DEBUG_INFO
+	help
+	  The implicit default version of DWARF debug info produced by a
+	  toolchain changes over time.
+
+	  This can break consumers of the debug info that haven't upgraded to
+	  support newer revisions, and prevent testing newer versions, but
+	  those should be less common scenarios.
+
+config DEBUG_INFO_DWARF4
+	bool "Generate DWARF Version 4 debuginfo"
+	select DEBUG_INFO
+	help
+	  Generate DWARF v4 debug info. This requires gcc 4.5+ and gdb 7.0+.
+
+	  If you have consumers of DWARF debug info that are not ready for
+	  newer revisions of DWARF, you may wish to choose this or have your
+	  config select this.
+
+config DEBUG_INFO_DWARF5
+	bool "Generate DWARF Version 5 debuginfo"
+	select DEBUG_INFO
+	depends on !CC_IS_CLANG || (CC_IS_CLANG && (AS_IS_LLVM || (AS_IS_GNU && AS_VERSION >= 23502)))
+	help
+	  Generate DWARF v5 debug info. Requires binutils 2.35.2, gcc 5.0+ (gcc
+	  5.0+ accepts the -gdwarf-5 flag but only had partial support for some
+	  draft features until 7.0), and gdb 8.0+.
+
+	  Changes to the structure of debug info in Version 5 allow for around
+	  15-18% savings in resulting image and debug info section sizes as
+	  compared to DWARF Version 4. DWARF Version 5 standardizes previous
+	  extensions such as accelerators for symbol indexing and the format
+	  for fission (.dwo/.dwp) files. Users may not want to select this
+	  config if they rely on tooling that has not yet been updated to
+	  support DWARF Version 5.
+
+config DEBUG_INFO_NONE
+	bool "Disable debug information"
+	help
+	  Do not build the kernel with debugging information, which will
+	  result in a faster and smaller build.
+
+endchoice # "Debug information"
 
 if DEBUG_INFO
 
@@ -267,55 +335,11 @@ config DEBUG_INFO_SPLIT
 	  to know about the .dwo files and include them.
 	  Incompatible with older versions of ccache.
 
-choice
-	prompt "DWARF version"
-	help
-	  Which version of DWARF debug info to emit.
-
-config DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
-	bool "Rely on the toolchain's implicit default DWARF version"
-	help
-	  The implicit default version of DWARF debug info produced by a
-	  toolchain changes over time.
-
-	  This can break consumers of the debug info that haven't upgraded to
-	  support newer revisions, and prevent testing newer versions, but
-	  those should be less common scenarios.
-
-	  If unsure, say Y.
-
-config DEBUG_INFO_DWARF4
-	bool "Generate DWARF Version 4 debuginfo"
-	help
-	  Generate DWARF v4 debug info. This requires gcc 4.5+ and gdb 7.0+.
-
-	  If you have consumers of DWARF debug info that are not ready for
-	  newer revisions of DWARF, you may wish to choose this or have your
-	  config select this.
-
-config DEBUG_INFO_DWARF5
-	bool "Generate DWARF Version 5 debuginfo"
-	depends on !CC_IS_CLANG || (CC_IS_CLANG && (AS_IS_LLVM || (AS_IS_GNU && AS_VERSION >= 23502)))
-	depends on !DEBUG_INFO_BTF
-	help
-	  Generate DWARF v5 debug info. Requires binutils 2.35.2, gcc 5.0+ (gcc
-	  5.0+ accepts the -gdwarf-5 flag but only had partial support for some
-	  draft features until 7.0), and gdb 8.0+.
-
-	  Changes to the structure of debug info in Version 5 allow for around
-	  15-18% savings in resulting image and debug info section sizes as
-	  compared to DWARF Version 4. DWARF Version 5 standardizes previous
-	  extensions such as accelerators for symbol indexing and the format
-	  for fission (.dwo/.dwp) files. Users may not want to select this
-	  config if they rely on tooling that has not yet been updated to
-	  support DWARF Version 5.
-
-endchoice # "DWARF version"
-
 config DEBUG_INFO_BTF
 	bool "Generate BTF typeinfo"
 	depends on !DEBUG_INFO_SPLIT && !DEBUG_INFO_REDUCED
 	depends on !GCC_PLUGIN_RANDSTRUCT || COMPILE_TEST
+	depends on !DEBUG_INFO_DWARF5
 	help
 	  Generate deduplicated BTF type information from DWARF debug info.
 	  Turning this on expects presence of pahole tool, which will convert
@@ -584,21 +608,6 @@ source "lib/Kconfig.kcsan"
 
 endmenu
 
-config DEBUG_KERNEL
-	bool "Kernel debugging"
-	help
-	  Say Y here if you are developing drivers or trying to debug and
-	  identify kernel problems.
-
-config DEBUG_MISC
-	bool "Miscellaneous debug code"
-	default DEBUG_KERNEL
-	depends on DEBUG_KERNEL
-	help
-	  Say Y here if you need to enable miscellaneous debug code that should
-	  be under a more specific debug option but isn't.
-
-
 menu "Memory Debugging"
 
 source "mm/Kconfig.debug"
-- 
2.30.2

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